Photographic printing apparatus



Sept. 13, 1960 E. c. ROGERS, JR 2,952,780

' PHOTOGRAPHIC PRINTING APPARATUS Filed Jan. 8, 1954' 2 Sheets-Shee t 1X 33- INVENTOR.

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c ATTORNEYS- Sept. 13, 1960 E. c. ROGERS, JR 2,952,780

PHOTOGRAPHIC PRINTING APPARATUS Filed Jan. 8, 1954 2 Sheets-Sheet 2INVENTOR. [in 0gp CPoeMs Jk i ORA/5Y5.

United States Patent ce PHOTOGRAPHIC PRINTING APPARATUS Elwood C.Rogers, Jr., 2927 N. Tibbs Ave., Indianapolis, Ind.

Filed Jan. 8, 1954, Ser. No. 402,962

9 Claims. (Cl. 250-214) This invention relates to methods and apparatusfor producing photographic enlargements and more particularly to anautomatic means for timing :an exposure in accordance with the intensityof the image projected on to the printing paper. Such automatic timingmeans for photo-enlarging or other photo-printing devices are notbroadly new. One form of such means previously used embodies aphotoelectric cell or other light-sensitive element which receives lighttransmitted through the printing paper and which functions to control inone way or another the duration of an exposure. Such a timing means isopen to the objection, among others, that it is susceptible to thelight-transmitting qualities of localized areas of the printing paper,and such light-transmitting qualities may vary from sheet to sheet oreven over the area of a single sheet. Other prior automatic timing meanshave been open to other objections.

It is an object of my invention to produce an improved method andapparatus for automatically timing photoprinting exposures. Moreparticularly, it is my object to provide an automatic timing method andapparatus which will accurately control the duration of the exposure inproportion to the intensity of the projected image to be printed andwhich will not be adversely affected by other variables. Another objectof my invention is to produce a timing apparatus which can be adjustedto vary the slope of the curve expressing the relationship betweenimage-intensity and exposure-time.

In carrying out my invention, I control the duration of exposure inaccordance with the time required to discharge (or charge) a timingcondenser over a circuit whose time constant varies with the intensityof the image being printed. The apparatus is adjustable to vary thecharge on the condenser in accordance with the printing characteristicsof the sensitized paper employed; and such condenser is arranged todischarge (or charge) through a photoelectric cell or otherlight-responsive element responsive to image-intensity. In the preferredform of the invention, as embodied in an enlarger, the image-intensityis measured by light reflected at a predetermined angle from a selectedportion of the image as projected on the printing paper. In associationwith the timing condenser and the light-responsive element, I provide ameans for controlling variation of the time constant of the circuit overwhich the condenser discharges (or charges), thus making it possible tovary the slope of the curve expressing the relationship betweenimage-intensity and exposure-time.

The accompanying drawings illustrate the invention:

Fig. 1 is an elevational view, somewhat diagrammatic in character,illustrating a photographic enlarger and, in association therewith, alight-responsive device embodying my invention;

Fig. 2 is a diagrammatic view illustrating a timing circuit in which theduration of the exposure is controlled by the time required to dischargea condenser; and

Fig. 3 is a diagram of another form of timing circuit 2,952,780 PatentedSept. 13, 1960 in which the duration of exposure is controlled by thetime required to charge a condenser.

The enlarging apparatus illustrated in Fig. 1 comprises a projectionlamp 10, a condensing reflector 11 therefor, a correctivelight-diffusing screen 12, and a projecting lens 13. The negative 14 tobe enlarged is supported in a suitable support between the diffuser 12and the lens 13 so that an enlarged image will be projected onto thesurface of a sheet of sensitized paper 15 on an appropriate support 16.The parts described, or their equivalents, are common to allphotographic enlargers and may be varied without departing from myinvention.

The preferred light-responsive device employed in practicing myinvention comprises a base 17 adapted to rest on the support 16 andprovided with a stem 18 which projects upwardly, conveniently at anangle to the vertical. Adjustably mounted on the stem 18 is a housing 19containing a photoelectric cell 20 mounted on the axis of a condensinglens 21. Behind the photoelectric cell 20 is a groundglass screen 22 orits equivalent, and behind the screen 22 there is mounted a lamp 23. Theangular disposition of the housing 19 is such that light reflected fromthe surface of the sensitized paper 15 will be concentrated by the lens21 on the cathode of the photoelectric cell 20. Desirably, the stem 18and housing 19 are so arranged that while the housing can be adjustedalong the stem 18 its angular disposition relative to the surface onwhich the base 17 rests will remain a constant.

In the use of the light-responsive device, a focused image of thenegative 14 is projected onto the surface of the paper-support 16 oronto the surface of an unsensitized white paper on such support. Withthe lamp 23 turned on, the light-responsive device is moved around theperiphery of the image, and the casing 19 is adjusted on the stem 18,until the image of the cathode of the photoelectric cell 20 covers anarea which is regarded as representative in tone quality. This area, theselection of which rests with the operator, is preferably a medium gray.Depending on the character of the image on the negative 14, the area maybe of uniform tone throughout or may be made up of properly balancedlight and dark portions. If desired, the lamp 23 and its housing may beomitted and the position of the light-responsive device selected by thelocation of the cathode-shadow with respect to the image appearing onthe ground-glass 22. In either arrangement, the desideratum is that thelight condensed by the lens 21 on the cathode of the photo-electric cell20 comes from :an area of predetermined average tone quality in theimage projected by the lens 13 of the enlarger. It is also desirablethat in any one apparatus the axis of the light-responsive device be ata fixed angle to the surface of the support 16; for the intensity oflight reflected from the surface of the sensitized paper will dependupon the angle of reflection.

The light-responsive unit above described is employed to control theoperation of a timing circuit such as is illustrated in Fig. 2 or Fig.3. The circuit of Fig. 2 comprises a pair of electron discharge devices,such as the pentodes 30 and 31, the cathode heating filaments of whichare supplied with current from a step-down transformer 32 the primary ofwhich is connected across an alternating-current supply line 3334,conveniently providing the common llO-volt, oil-cycle current. One ofthe supply conductors, shown as the conductor 33, is directly connectedto the cathode of the pentode 30 and connected to the cathode of thepentode 31 through a variable resistance 36. The anode or plate of thepentode 31 is connected to the other supply conductor 34 through thewinding of a normally open holding relay 38 and a fixed resistance 39.The plate of the pentode 30 is connected to the conductor 34 through theresistance 39, a second fixed resistance 40, and a potentiometer 41having an adjustable tap 42. The control grid of the pentode 31 isdirectly connected to the plate of the pentode 30, while the controlgrid of the pentode 30 is connected to one terminal of a timingcondenser 43.

The projecting lamp is under the control of a relay 45 having a movablecontact 46 and a fixed contact 47 separated from each other when therelay is de-energized. One terminal of the winding of the relay 45 isconnected to the supply conductor 34, while the other terminal isconnected to the supply conductor 33 through the normally separatedcontacts of the holding relay 38 and also through a normally open,push-button switch 48 in parallel with the contacts of the relay 38. Topermit use of the projecting lamp 10 for set-up purposes, a manuallycontrolled switch 49 may be connected in parallel with the contacts 46and 47 of the relay 45.

As previously indicated, one terminal of the timing condenser 43 isconnected to the control grid of the pentode 30. The other terminal ofsuch condenser is connected to the movable contact 51 of a single-pole,double-throw switch the two stationary contacts 52 and 53 of which areconnected respectively to the tap 42 of the potentiometer 41 and to thecathode of the pentode 30. The dotted line 54 in Fig. 2 indicates amechanical interconnection between the movable contact 51 and themovable contact 46 of the relay 45, the arrangement being such that themovable contact 51 engages the fixed contact 52 when the relay 45 isde-energized and engages the fixed contact 53 when the relay 45 isenergized.

Connected in parallel across the resistances 40 and 41 is apotentiometer 56 having an adjustable tap 57 connected to the anode ofthe photoelectric cell 20. The cathode of the photoelectric cell isconnected to the control grid of the pentode 30. For a purpose whichwill hereinafter become apparent, a manual, normally open push-buttonswitch 58 is connected across the terminals of the timing condenser 43.

The lamp 23 associated with the light-responsive element is shown inFig. 2 as connected to the supply conductors 33 and 34 through a switch60. The projecting lamp 10 is connected to the supply wires 33 and 34through the parallel switches 4647 and 49.

The parts are illustrated in Fig. 2 in the condition existing during theinterval between exposures. In this condition, the relays 45 and 38 arede-energized, and the movable contact 51 is in engagement with the fixedcontact 52 to connect the condenser 43 between the grid of the pentodeand the adjustable tap 42 of the potentiometer 41. Since the plates ofboth pentodes are connected to the same supply conductor 34 bothpentodes will be non-conductive during one-half of each cycle of thesupply current; and sofar as operation of the pentodes is concerned,they may therefore be regarded as subjected to a direct-current platevoltage. The pentode 30 will be conducting, depressing the potential ofits plate and that of the grid of pentode 31. The resistance 36 is greatenough to insure that the grid of pentode 31 will possess a negativebias great enough to render that tube non-conductive, and thus maintainthe relay 38 de-energized as long as the tube 30 is conductive.

It will be clear from Fig. 2 that in the condition there illustrated thepotential diiterence between the control grid of the pentode 30 and thetap 42 of the potentiometer 41 will be applied across the timingcondenser 43 to impart a charge thereto and that the potential to whichthe timing condenser is charged Will depend upon the position of the tap42. Assuming that the apparatus has been adjusted to take account of theprinting qualities of the paper being employed, that thelight-responsive device 2021 has been properly positioned, and thateverything is in readiness for the making of an enlargement, printingoperation is started by closing the push-button switch 48. Thisoperation completes a circuit through the winding of the relay 45,closes the switch 4647, and causes the projecting lamp 10 to light.Energization of the relay 45 also moves the movable contact 51 fromengagement with the contact 52 into engagement with the contact 53. Suchmovement of the contact 51 connects the timing condenser 43 between thecathode and grid of the pentode 30 in such a way that the grid isnegatively biased and the pentode 30 thereby rendered non-conductive.This causes an immediate rise in the potential of the plate of thepentode 30 and of the grid of the pentode 31. The pentode 31 thereuponbecomes conductive, and the resultant current flowing through the relay38 causes the contacts of such relay to close and make it possible forthe operator to release the push-button switch 48 in parallel with suchcontacts, the relay 45 remaining energized and the projecting lamp 10 inoperation. The condenser 43 now discharges through the photoelectriccell at a rate dependent upon the amount of light received by such cell;and as the condenser discharges, the negative bias of the grid of thepentode 30 decreases. Eventually, such bias becomes small enough torender the pentode 30 conductive, whereupon the potential of its plateand of the grid of the pentode 31 decreases, the pentode 31 becomesnon-conductive, the relay 38 is de-energized to de-energize the relay45, and the contacts 46 and 47 of the latter relay separate toextinguish the projecting lamp 10 and terminate the printing operation.

The position of the tap 42 along the resistance 41 determines the chargeimpressed on the timing condenser 43 during intervals between exposures,and the tap 42 may therefore be adjusted to suit the speed of thesensitized paper being used in the production of enlargements. In orderto prevent the charge impressed on the condenser 43 during one settingof the tap 42 from influencing operation of the device afterreadjustment of such tap, the push-button switch 58 is momentarilyclosed to discharge the condenser 43 after each tap-readjustment.

It will be noted that the potentiometer 56 forms part of a circuit whichincludes the timing condenser 43 and photoelectric cell 20 and which isin parallel with the pentode 30. During a timed exposure, the voltagedrop across this potentiometer will be slowly increasing as thecondenser 43 discharges, the rate of change in such voltage dropdepending upon the intensity of light striking the photoelectric celland upon the setting of the tap 57. As a result, the positive potentialapplied to the anode of the photoelectric cell will be decreasing andwill reduce the time constant of the circuit over which the condenserdischarges. Obviously, the rate at which such time constant varies willdepend upon the setting of the tap 57. Specifically, the rate of changeof the potential applied to the anode of the photoelectric cell willincrease as the tap '57 is moved toward that terminal of the resistance56 which is connected to the plate of pentode 30. The greater the changein potential applied to the anode of the photoelectric cell, the longerwill be the long exposures as compared with the short exposures, andvice versa. Hence, the potentiometer 56 enables the operator to controlthe slope of the curve representing the relation between light-intensityand exposure-time and thus compensate for such variables as gridcurrent, condenser leakage, wiring leakage, and gas currents in thephotoelectric cell and in the pentode 30.

The variable resistance 36 controls the cathode-potential, and hence theeifective grid-potential, of the pentode 31. Its adjustment provideslarge current and voltage changes for the pentode 31 and can be employedto effect large changes in light-time values by shifting the operatingcharacteristics of the circuit.

Desirably, the heating-filament circuit of the pentode 30 includes aresistance 62 which maintains filament heat, and thus grid and gascurrents in the pentode 38, at a low value so as to reduce the magnitudeof the factors compensated for by adjustment of the potentiometer 56.

To increase the range of adjustment, it may be desirable in someinstances to vary the capacity of the timing condenser. To this end, asecond condenser 43 may be connected in parallel with the condenser 43through a switch 63 which can be closed or opened as necessary toincrease or decrease the effective capacity of the timing condenser. Ifthe voltage across the supply conductors 33 and 34 is subject tovariation, it is desirable to employ a voltage-regulator tube 64connected across such conductors. The degree of voltage provided by thetube 64 is controlled by the valve of the resistance 39.

It is common in photo-enlarging work to employ a so called safe lightwhich projects light of a wave length which is visible but to whichprinting papers are not responsive. In ordinary work, such a safe lightcan be left on during a printing operation. With apparatus embodying myinvention, however, it is advisable to extinguish the safe light duringa printing operation because, although the sensitized paper may not beaffected by the wave lengths it emits, the photoelectric cell 20 mightbe. Automatic control of the safe light may be conveniently effectedthrough the expedient illustrated in Fig. 2, where a safe light 70 isshown as connected to the supply conductors 33 and 34 through a switchcomprising the movable contact 46 of the relay 47 and a fixed contact 71which such movable contact engages when the relay 45 is de-energized.With this arrange ment, when the relay is energized to institute aprinting operation, the safe light is automatically turned out as aresult of operation of the contacts 46 and 71.

To set the device up for operation, a negative is positioned in theenlarger, the projection lamp turned on by closing the manual switch 49,and the enlarger is adjusted to project onto the surface of the support16 a focused image of the desired size. The light-responsive controldevice is then positioned with respect to the image in the manner abovedescribed so that reflected light from an image-portion of thepredetermined tone quality will be projected onto the cathode of thephotoelectric cell 20. If the tap 42 of the potentiometer 41 is notproperly adjusted for the speed of the printing paper to be used, itsadjustment is appropriately changed and the push-button switch 58momentarily closed to discharge the condenser 43 and thus insure thatwhen recharged its potential will correspond to the new setting of thetap 42. With the manual switch 49 to extinguish the projecting lamp, theapparatus is in condition for the making of an exposure. With the aid ofillumination provided by the safe light, which is connected to thesupply conductors 33 and 34 through the interengaged contacts 46 and 71,the operator positions the printing paper on the support 16 andmomentarily closes the pushbutton switch 48 whereupon the apparatusoperates automatically to extinguish the safe light 70 and initiate anexposure by turning on the projection lamp 10. Upon termination of theexposure period, marked by the deenergization of the relays 38 and 45 asabove set forth, the projection lamp is extinguished and the safe light70 again turned on.

In making a plurality of enlargements from a single negative, no furtheradjustment of the apparatus is required. It is only necessary to removethe exposed paper, position the new sheet, and again close momentarilythe push-button switch 48. The minimum possible interval betweenexposures is much greater than that necessary to charge the condenser 43fully. A change in negatives will ordinarily require a change in thepositioning of the light-responsive control device, and a change inpaper used may require a re-adjustment of the tap 42.

The only movable parts of the timing circuit which need be exposed forordinary manual manipulation are a control for the tap 42 and theoperating members of the push-button switches 48 and 58. The adjustmentsfor the variable resistances 36 and 56 need not be, and desirably arenot,v readily accessible; forv those adjustments need be made only atinfrequent intervals, as to compen sate for aging of the tubes. I

I am aware that it is old to time intervals by the d scharge of acondenser associated with an electron 1118- charge device, such as thepentode 30. In many of such prior devices, however, it is necessary toperform some manual operation, such as the opening of a switch, afterthe termination of the automatically timed interval in order to preventthe condenser from recharging and automatically instituting a newoperation. When such a device is used to control a printing exposure,adventitious recharging of the condenser would result in a re-exposurewhich might be highly undesirable. No such adventitious re-exposure canoccur in the use of my device; since the operation of the switch 51 andthe control of an automatically timed interval prevents the rechargingof the condenser from instituting a new operation.

While the apparatus above described is primarily suited for the timingof exposures in photo-enlarging, the circuit illustrated in Fig. 2 canbe used in other situations, as the source of light which controls thephotoelectric cell 20 is immaterial. It will also be understood thatwhile I have illustrated in Fig. 2 a circuit specifically adapted forconnection to an alternating-current supply the apparatus can beadapted, by obvious changes, for use with a direct-current supply.

In the timing circuit illustrated in Fig. 3, the simple photo-electriccell 20 has been replaced by a photomultiplier 80. Further, the circuitis such that the duration of the exposure-interval is controlled by thetime required to charge the timing condenser to a predetermined valuerather than by the time required to discharge it.

Voltage for the photo-multiplier is supplied by a step-up transformer81, the voltage-divider 82 of the tube 80 being connected across thesecondary 83 of the transformer 81 in series with a variable resistance84 in such a manner that the resistance 84 will control the voltageacross the last stage of the cell 80. In addition to the secondary 83,the transformer 81 may include a lowvoltage secondary winding 86providing current for the filament of an electron-discharge tube 87. Toavoid undue complication of the drawing the connection of the winding 86and such tube-filament is not illustrated.

The primary of transformer 81 is connected across the ordinary 110 voltsupply line 33, 34 preferably through a variable resistor 90. Thecathode of tube 87 is connected to an extension 34 of the supply line 34through a variable resistance 91 which controls the efiectiveplatevoltage of tube 87. The anode of tube 87 is connected to anextension 33 of the supply line 33 through the winding of the relay 38,a normally closed switch 92, and a resistor 93, the latter serving as apower resistor to eliminate excessive line surges on relay 38. Thecontacts of relay 38 are normally open and are connected in series withthe winding of a second relay 94 between the conductors 33' and 34, sothat energization of the relay 38 will elfect energization of the relay94.

The relay 94 has two movable contacts 95 and 96. The contact 95, whichis connected to the conductor 34', engages a fixed contact 97 when therelay 94 is deenergized and a fixed contact 98 when such relay isenergized. The contact 97 is connected through the safe-light 70 withthe conductor 33 while the contact 98 is connected through the enlargerlamp 10 with that same conductor. As will be obvious from thearrangement of relay contacts just described, the safe light 70 will belit and the enlarger lamp 10 out when the relay 94 is deenergized, whilethe enlarger lamp 10 will be lit and the safe light out when the relayis energized.

Co-operating with the second movable contact 96 of the relay 94 are twofixed contacts 100 and 101, the former engaged by the movable contact 96when the relay 94 is de-energized and the latter engaged by such movablecontact when the relay is energized. The fixed con 7 tact 100 isconnected to the adjustable tap of a potentiometer 102 which isconnected in series with a rectifier. 103- between the conductors 33'and 34, the rectifier being-arranged to pass current fromthe conductor33 to the conductor 34'. The stationary contact 101 is connected througha variable resistance 104 to the anode of the photo-multiplier tube 80and through a resistor 105 to the control element of the tube 87. Theresistor 105.

is included in the lead to the control element of tube 87 as aprecaution to prevent excessive grid current in the timing circuit.

A timingcondenser107 is connected between the relay contacts 96 and 100in parallel with a variable resistance 108. One or more auxiliary timingcondensers 107' adapted for selective connection, as through one or moreswitches 106, in parallel with the condenser 107 may be provided toincrease the range of timing intervals which the apparatus provides.

In addition to the main elements so far identified, the circuit of Fig.3 desirably includes certain additional elements designed to improve orfacilitate operation. These additional elements include a condenser 110connected across the winding of relay 38 to reduce pulsations in thehalf-cycle rectified plate current of tube 87 whereby to keep relay 38from chattering. A condenser 111 and a resistance 112 connected inseries with each other across the winding of the relay 94 are balancedto reduce arcing at the contacts of relay 38 due to inductive kicksfromthe winding of the 1l0-volt relay 94. The manually operable switch49, which can be closed to provide for the supply of current to theenlarger lamp for purposes of focusing, is connected across the relaycontacts 95 and 98. A normally open initiating switch 48 is connected inparallel with the normally open contacts of relay 38.

The circuit illustrated in Fig. 3 operates as follows: Normally, therelays and switches are in the respective conditions illustrated in thefigure. The relay contacts 95 and 97 are in engagement, thus causingcurrent to be supplied to the safe light 70, while the circuit throughthe enlarger lamp 10 is open at 9895. The relay contact 96 is inengagement with the fixed contact 100, thus short-circuiting the timingcapacity 107 and insuring that there will be no voltage across suchcapacity upon the initiation of an exposure. Enough dark current willflow in the tube 80 to maintain on the control element of tube 87 anegative bias suflicient to insure de-energization of.

the relay 38.

For the purpose of focusing the enlarger, the. switch 49 is closed, thuscausing current to be supplied to the enlarger lamp 10. At the sametime, and as set forth in connection with the circuit shown in Fig. 2,the lightresponsive unit is positioned to be responsive to lightreflected from the selected portion of the image projected by theenlarger. The enlarger having been properly focused and the lightresponsive unit properly adjusted, the switch 49 is opened to turn outthe enlarger lamp 10, the printing paper is arranged on the easel of theenlarger, and the switch 48 is momentarily closed to complete a circuitfrom the conductor 34' through the Winding of relay 94 to the conductor33'. The resultant energization of therelay .94 moves the contact 95 toturn off the safe light 70 and turn on the enlarger lamp 10. Concurrentmovement of the movable relay contact 96 connects the timing capacity107, through the variable resistance 104., between the anode of tube 80and the adjustable tap of the potentiometer 102. Because of therectifier 103, the adjustable tap of the potentiometer 102 will bepositive with respect to -the cathode of tube 87, and since there is noinitial charge on the condenser 107, engagement of the contact 101 bythe contact 96 will place an eifective positive potential on the controlelement of tube 87, thus rendering such tube conductive and causing therelay 38 to be energized. Resultant closure of the relay contacts 38will complete a connection between the conductor 34 and the winding ofrelay 94, so that upon subsequent opening of the switch 48; relay 94;will remain energized. Current flowing in the photo-multiplier tube'will gradually charge the condenser 107, tending to. make the con-'trol element of; the tube 8.7negative'with respect to the adjustable tapof potentiometer 102. Eventually, the. negative potential acrosscondenser .107 will rise to such a value that the tube 87 becomesnon-conductive. When thishappens, the relay 38.will be de-energized,thus opening the circuit through the relay 94. The resultantdeenergization of the relay 94 extinguishes the enlarger lamp 10,.lights the safe. light;.70,., and discharges the timing condenser 10.7.An exposure can be terminated at any time merely by" a momentary openingof switch 92, thusthe start of the timing cycle and the greater will bethecharge. which must be builtup on the timing condenser to render tube87 non-conductive. Accordingly, adjustment of the top of potentiometer1.02 away from conductor 34' will increase the exposure. interval andadjustment toward such conductor will shorten the exposure, theintensity of the image projected. on the paper remaining the same.

For wider variations, in paper speed, the timing capacity may be variedby adding or substracting supplementary condensersv 107 It is sometimesdesirable to provide a stop, such as an iris diaphragm (not shown), inassociation with the lens 21 of the lightsensitive unit in order to varythe selectivity of that portion of the projected picture which is beingused as a control area. Stopping the lens 21 will obviously reduce thelight incident on the photo multiplier cell, which would increase theduration of the automatically timed exposure interval; but by increasingthe magnitude of the timing capacity 107, 107' in an amount coordinatedwith. the stopping of the lens, the exposure interval can be keptconstant.

Variations between dilferent'photocells 80, or variation in theperformance of any one cell with aging, may require adjustments otherthan those mentioned above as for the purpose of accommodating forvariations in paper speed or in the aperture of lens 21. Suchadjustments are provided by the variable resistor in the primary circuitof transformer 81, by the resistor 84 which is connected across thefinal stage of the photocell 80, by the resistor 104 in the lead to theanode of the photocell, and by the resistor 108 in parallel with the.timing capacity. Primarily, these variable resistors alter the slope ofthe lighttime curve characteristic of the device. creasing theresistance of the resistor 90, it is possible to reduce the voltageapplied to the photocell and thereby to reduce its multiplying factor.The resistance 104 has a greater proportionate effect when strong lightis striking the cell 80 than. when the light striking such cell is weak.The resistance 108 provides a leakage path for the current whichchargesthe timing capacity. This resistance permits a substantially constantleakage current, and hence aifeots the long exposures proportionatelymore than it does the short exposures. Varying the value of theresistance 84, by varying the potential across the last stage of thecell 80, likewise affects the long exposures more than it does the shortones. By appropriate adjustment of the several resistances mentioned, orany of them, it is possible to so set the device that the light receivedby the cell 80 will be an effective measure of the proper exposureinterval.

- With both of the circuits shown in Figs. 2 and 3 it is contemplatedthatthe safe light 70 will be extinguished The farther the tap of Thus,by in-' during exposure. Some operators may desire to leave the safelight on at all times, as by connecting it to the conductor 34'independently of the relay contacts 95, 97. The most commonly used safelight for use with enlargers is yellow in color; and since photocells,as a general rule, are relatively more sensitive to yellow light thanare printing papers, the duration of the exposure will be shortened ifthe safe light is left on during the operation. To prevent the safelight from thus aifecting exposure duration, I may provide inassociation with the photocell a blue filter 20 (Fig. 1) to preventreflected light from the safe light from impinging on the cell andshortening the exposure.

By way of illustration and not limitation, I set forth belowspecifications for the more important components of the circuit shown inFig. 3, it being assumed that the supply is the customary 110-volt,60-cycle supply:

Photomultiplier 80 Type 93 l-A. Transformer 81 Plate-type powertransformer, 650 volt, 40 milliamperes.

Voltage divider 82 Nine segments, 0.47

megohms each.

Resistor 84 -5 megohms.

Tube 87 6J5.

Resistor 90 0l000 ohms.

Resistor 91 0-1000 ohms.

Resistor 93 1000 ohms.

Resistor 102 50,000 ohms.

Rectifier 103 35 milliamperes.

Resistor 104 010 megohms.

Resistor 105 1.0 megohm.

Condensers 107, 107 To provide incremental capacities from 0.1-4.0 mfd.

Resistor 108 0-200 megohms.

Condenser 110 20 mfd.

Condenser 111 1.0 mfd.

Resistor 112 600 ohms.

Relay 38 Plate-type, 2500 ohms.

This application is a continuation-in-part of my prior applicationSerial No. 83,897 filed March 28, 1949, now Patent No. 2,668,474,granted February 9, 1954, in which certain features of the describedapparatus are claimed.

I claim as my invention:

1. In a device for automatically terminating a photographic exposure, anelectron discharge device having a control element and an associatedanode circuit currentflow in which is controlled by the potential ofsaid control element, a first relay in said anode circuit havingnormally open contacts, a second relay, a holding circuit including thecontacts of said first relay and the winding of said second relaywhereby de-energization of the first relay will cause de-energization ofthe second relay, timing means for controlling the potential of saidcontrol element, said timing means comprising a timing.

circuit including a condenser and a light-responsive means for varyingcurrent in the timing circuit and coordinately varying the rate at whichthe potential of said control element decreases, and means operativeupon de-energization of said second relay for terminating an exposure.

2. A device as set forth in claim 1 with the addition that saidcondenser is connected to said control element whereby the potential ofthe control element will be determined by the charge in the condenser.

3. A device as set forth in claim 1 with the addition of a secondelectron discharge device having an anode connected to the controlelement of the first electron discharge device and a control elementconnected to said condenser so that as the charge on said condenserdecreases anode current in the second electron discharge device willdecrease to cause an increase in the potential of the control element ofthe first electron discharge de- 10 vice, and means for preliminarilyimposing a charge of predetermined magnitude on said condenser.

4. In a device for automatically terminating a photographic exposure, anelectron discharge device having a control element, a cathode, and anassociated anode circuit current-flow in which is controlled by thepotential of said control element, a timing condenser connected betweensaid cathode and control element whereby the potential of the controlelement will be proportional to the charge on the condenser, acondenser-charging circuit, light-responsive means in saidcondenser-charging circuit for varying the rate of condenser-charging,exposure-terminating means in said anode circuit, and a variableresistance in parallel with said condenser for bleeding oil a controlledportion of condenser-charging current during the exposure interval.

5. In a device for automatically terminating a photographic exposure, anelectron discharge device having a control element, a cathode, and anassociated anode circuit current-flow in which is controlled by thepotential of said control element, a timing condenser connected betweensaid cathode and control element whereby the potential of the controlelement will be proportional to the charge on the condenser, acondenser-charging circuit, light-responsive means in saidcondenser-charging circuit for varying the rate of condenser-charging,exposure-terminating means in said anode circuit, and a variableresistance in series with said condenser and lightresponsive means forregulating the condenser-charging current during the exposure interval.

6. In a device for automatically terminating a photographic exposure, anelectron discharge device having a control element, a cathode, and anassociated anode circuit current-flow in which is controlled by thepotential of said control element, a timing condenser connected betweensaid cathode and control element and chargeable to increase negativebias on said control element, a condenser-charging circuit, aphotoelectric cell in said condenser-charging circuit for varying therate of condenser-charging, exposure-terminating means in said anodecircuit, and adjustable voltage-regulating means for varying theresponse of said photoelectric cell.

7. The invention of claim 6 with the addition that said photoelectriccell is a photomultiplier having a plurality of stages, said regulatingmeans being associated with and effective upon only the last stage ofsaid photomultiplier.

8. In a device for automatically terminating a photographic exposure, anelectron discharge device having a control element, a cathode, and anassociated anode circuit current-flow in which is controlled by thepotential of said control element, a timing condenser connected betweensaid cathode and control element and chargeable to increase negativebias on said control element, a condenser-charging circuit, aphotomultiplier in said condenser-charging circuit for varying the rateof condenser-charging, exposure-terminating means in said anode circuit,and adjustable means for varying the aflect of said photomultiplier uponthe charging of the condenser.

9. Apparatus for use in a region illuminated by a colored safe light,comprising a photographic enlarger having a light-source and adapted toproject an image for printing on a sensitized surface, automatic timingmeans, exposure-terminating means controlled by said timing means, saidtiming means including a light-responsive element positioned to receivelight from said projected image and operative to vary the duration ofthe exposure inversely with respect to the intensity of the light itreceives, and a light filter positioned in association with saidlight-responsive element to intercept all light falling thereon, saidfilter having a color complementary to that of said safe light wherebyit will prevent light from the safe light from affecting saidlight-responsive element but will permit light of its own color receivedfrom said projected image to fall upon the light-responsive element.

References Cited in the file of this patent UNITED STATES PATENTS 12Burnham et a1 July: 11, 1944- Simmon Mar. 23,1948 Rabinowitz May 3, 1949 Labrum Oct. 11, 1949 Austin July 11, 1950 Schwennesen Dec. 25, 1950Feller Oct. 6, 1953 Levine Jan. 19, 1954

